1. Field of the Invention
The present invention relates to an acetylene black having both high electrical conductivity and high absorptive capacity for liquids. The invention also relates to a process for the preparation of this acetylene black.
2. Description of the Prior Art
The conductive carbon blacks may be classed in two large categories:
(1) The acetylene blacks, which are characterized by a very high purity (carbon content over 99%) and by being highly structured, and therefore have high absorptive power for liquids and excellent electrical conductivity; and
(2) The furnace blacks, having undergone a thermal treatment, which are much less pure and less highly structured than the acetylene blacks, but which however do have good electrical conductivity.
The acetylene blacks are used primarily in the manufacture of electric cells. Their great absorptive power ensures retention of the electrolyte, and the black + electrolyte mixture simultaneously maintains a solid appearance and is thus capable of forming cylindrical blocks. The high purity and great electrical conductivity of the acetylene blacks enable them on the one hand to ensure the transport of current, and on the other hand, to exhibit a perfect chemical inertia towards the other constituents of the cell.
The thermally treated furnace blacks do not possess sufficient purity to be used in electric cells. They are used in the manufacture of conductive rubbers.
The acetylene black plays a dual role in the electric cell. On the one hand, due to its absorptive properties, it permits retention of the electrolyte, and on the other hand, due to its conducting properties, it gives the cell a weak internal resistance. In fact, when the black used is a black of high purity like an acetylene black, the amount of black necessary to ensure good electrical conductivity to the whole is much less than that necessary for the absorption of the electrolyte. There is thus seen the advantage to the user of possessing an acetylene black having the maximum absorptive power. The more highly absorptive the black is, the smaller will be the quantity of the black to be incorporated in the black + electrolyte mixture.
The acetylene blacks known up to now have an electrical resistivity, measured under a pressure of 6.3 bars, which is not less than 0.4 ohm.cm and an index of absorption of dibutyl phthalate (or DBP index) which does not exceed 320 ml. per 100 g of black. This DBP index, which is evaluated by the method of French standard NF T 45-122 modified as indicated hereinafter, is a measure of the absorptive power of the black for liquids.
These acetylene blacks are obtained either by thermal decomposition, at a temperature not more than 1800.degree. C., of acetylene or a mixture of hydrocarbons containing acetylene (see French Pat. Nos. 941,596; 1,021,995; and 1,462,071; and U.S. Pat. No. 2,121,463), or by partial combustion, at a temperature above 1700.degree. C., of a hydrocarbon other than acetylene (for example ethylene or benzene) by oxygen (see French Pat. No. 2,229,744). The decomposition temperature attained in the processes mentioned above, based on the thermal decomposition of acetylene or of mixtures containing acetylene, which temperature is also that of formation of the black and governs the structure and therefore the absorptive power of the black, is far removed from the theoretical temperature of formation of the black to which the equation (I) of thermal dissociation of acetylene leads: EQU C.sub.2 H.sub.2 .fwdarw.2C+H.sub.2 +54.2 KCal (1)
This theoretical temperature is in fact around 2500.degree. C.